Air brakes for trucks and the like are activated by a treadle valve with a relatively short stroke that is not affected by the brake chamber displacement. Consequently, it is relatively difficult for drivers to sense from the brake pedal travel the extent of out of adjustment of the brakes. Brake adjustment is extremely important because the push rod force drops off rapidly once the push rod travel exceeds approximately two inches.
One way of determining brake adjustment is to measure the brake rod travel by marking the brake rod at the brake chamber exit point while it is in the most retracted position. The driver then applies the brakes while an assistant measures the brake rod travel to determine if it is within safety limits. Since the brake rod is only visible from underneath the vehicle and is not protected from road grime, one actually has to physically get underneath the vehicle and clean the brake rod before one can determine the amount of travel of the brake rod. Thus, the test is relatively cumbersome and inconvenient to perform, which typically leads to relatively infrequent and irregular testing to be done.
Therefore, there is a need for an indicator for air brakes for vehicles that determines in a convenient and accurate way the extent of out of adjustment of the brakes to insure that they remain in good operating condition.